In many conventional image acquisition and image data processing systems, feature geometry of a known, highly accurate artifact may become distorted during imaging, data processing, or both. One such situation may arise where a precision Cartesian grid (or array) of points printed on glass or other substrate material is imaged using optics and a camera, such as a charge-coupled device (CCD) camera, for example, or a complementary metal-oxide semiconductor (CMOS) imaging device. Such artifact features may be referred to as fiducials, and the foregoing substrate having a known pattern of fiducials printed thereon, or incorporated into the structure thereof, may be referred to as a fiducial plate.
FIG. 1 is a simplified diagram illustrating raw image data acquired by an imaging apparatus and representing a top view of a precision Cartesian grid of fiducials printed on a fiducial plate. Image acquisition of such fiducial plates and fiducial arrays may have utility in various contexts such as semiconductor probe card testing processes, for example, calibration of high-resolution imaging apparatus, and other imaging applications requiring a high degree of accuracy.
Given a precision artifact such as a fiducial or a fiducial array or grid to be imaged, conventional technology is deficient to the extent that it lacks the ability to identify the non-linear transformation to and from imaged coordinates (i.e., coordinates derived from acquired image data) and artifact coordinates on the fiducial plate (i.e., actual coordinates of the fiducial relative to a reference point on the fiducial plate).
Specifically, in an acquired image (i.e., image data obtained by a camera or other imaging hardware), the respective location of the center of each respective fiducial may be extracted from the acquired image data. In the case where the fiducial plate carries a Cartesian grid of fiducials, ideally, the fiducial locations in the acquired image form a regular, known rectangular grid aligned, for example, with the axes of the camera or other imaging apparatus. Due to factors such as fiducial absence, stage rotations, camera rotations, pixel size variation, magnification variation, keystone/barrel distortion, and other optical or mechanical effects, the measured fiducial locations may deviate from the ideal regular rectangular grid (e.g., as it exists on the Cartesian array of the fiducial plate).